1. Field of the Invention
The present invention relates to systems and methods for testing wireless devices, and in particular to such systems and methods in which signals exhibit DC offsets requiring correction for proper testing.
2. Related Art
Many modern devices utilize wireless signals to send and receive data. Handheld devices in particular make use of wireless connections to provide features including telephony, digital data transfer, and geographical positioning. Although a variety of different wireless-connectivity capabilities are used (such as WiFi, WiMAX, and Bluetooth), in general each is defined by an industry-approved standard (such as IEEE 802.11, IEEE 802.16 and IEEE 802.15, respectively). In order to communicate using these wireless-connectivity capabilities, devices must adhere to the parameters and limitations specified by the associated standards.
Although differences exist between wireless communication specifications (for example, in the frequency spectra, modulation methods, and spectral power densities used to send and receive signals), almost all of the wireless connectivity standards specify the use of synchronized data packets to transmit and receive data. Furthermore, most devices adhering to these wireless communications standards employ transceivers to communicate; that is, they transmit and receive wireless radio frequency (RF) signals.
At any point along the device-development continuum, it may be necessary to test and verify that a device is operating according to the standards associated with its various communication capabilities. Specialized systems designed for testing such devices typically contain subsystems operative to communicate with a wireless communications device during a test. These subsystems are designed to test that a device is both sending and receiving signals in accordance with the appropriate standards. The subsystems must receive and analyze device-transmitted signals and send signals to the device that subscribe to industry-approved standards.
The testing environment generally consists of the device under test (DUT), the tester, and a computer. The tester is generally responsible for communicating with the DUT using a particular wireless communication standard. The computer and tester work together to capture a DUT's transmitted signals and then analyze them against the specifications provided by the underlying standard to test the DUT's transmission capabilities.
As is well known in the art, the time required to test a device has a linear relationship with the cost associated with conducting the test. Therefore, it is advantageous to reduce the amount of time required by a test, thereby increasing the throughput of each test system and lowering overall production costs. Several factors contribute to the total time required to test a device. These factors include the time spend handling a device, setting up the test, sending control signals from the tester to the device, capturing signals sent by the device, and analyzing those captured signals.
Test accuracy demands that the test system components responsible for determining signal frequencies and generating the various signal components are held to a high degree of accuracy, since these provide the references against which a DUT operating performance is determined. It is common in the art to perform calibrations of such components and their subsystems often enough to ensure that any imperfection in a mixer or preceding stage producing a DC offset is corrected by reducing or cancelling out such a DC offset. In some cases, test systems are programmed to perform a recalibration step whenever test frequencies are changed, since DC offset is usually frequency dependent. In many cases, though, such recalibration may be unwarranted because the DC offset is sufficiently low to preserve test accuracy within standard-specified limits.
Despite the advantages gained by reducing overall test time, the accuracy and validity of a test cannot be compromised. At a minimum, doing so would increase the rate of retesting necessary to evaluate a device, thereby increasing the total time required for testing. As such, methods for decreasing the time required to perform a test without eliminating necessary steps or compromising the integrity of the test are desired.